pH Regulates White-Opaque Switching and Sexual Mating in Candida albicans

Abstract

As a successful commensal and pathogen of humans, Candida albicans encounters a wide range of environmental conditions. Among them, ambient pH, which changes frequently and affects many biological processes in this species, is an important factor, and the ability to adapt to pH changes is tightly linked with pathogenesis and morphogenesis. In this study, we report that pH has a profound effect on white-opaque switching and sexual mating in C. albicans. Acidic pH promotes white-to-opaque switching under certain culture conditions but represses sexual mating. The Rim101-mediated pH-sensing pathway is involved in the control of pH-regulated white-opaque switching and the mating response. Phr2 and Rim101 could play a major role in acidic pH-induced opaque cell formation. Despite the fact that the cyclic AMP (cAMP) signaling pathway does not play a major role in pH-regulated white-opaque switching and mating, white and opaque cells of the cyr1/cyr1 mutant, which is defective in the production of cAMP, showed distinct growth defects under acidic and alkaline conditions. We further discovered that acidic pH conditions repressed sexual mating due to the failure of activation of the Ste2-mediated α-pheromone response pathway in opaque A: cells. The effects of pH changes on phenotypic switching and sexual mating could involve a balance of host adaptation and sexual reproduction in C. albicans.

FIG 1

pH regulates white-to-opaque switching in air and in 5% CO₂. An α/α strain (FC4, a stock of WO-1 [15]) was used for this experiment. White cells were plated onto Lee's glucose medium plates and incubated at 25°C for 7 days. Quantitative assays of the switching frequencies are shown in Table 1.

FIG 2

pH has no obvious effect on opaque-to-white switching. (A) Images of strain GH1012 (a/a) colonies at different pHs. (B) Quantitation of the frequencies of opaque-to-white switching. Four genetically independent strains were used. Opaque cells were plated onto Lee's glucose medium plates and incubated at 25°C in air for 7 days.

FIG 3

pH regulates the induction of MFA1 expression in response to α-pheromone (produced by opaque α cells). A GFP-encoding sequence was integrated into the MFA locus and used as a reporter of the MFA1 promoter. Opaque α cells of GH1349, which served as an α-pheromone producer, were mixed with opaque a cells of the P_MFa1-GFP reporter strain. Single-strain cultures (GH1349 and the reporter strain) served as controls. Cells were incubated in liquid Lee's glucose medium with shaking at 25°C for 24 h. DIC, differential interference contrast.

FIG 4

Relative expression levels of MFA1 (A) and STE2 (B) in response to α-pheromone under different pH conditions. The WT (GH1013) control and the ste11/ste11, hst7/hst7, and rim101/rim101 mutants are all SC5314 derivatives. To generate the MFA1 reporter strain SZ306a P_MFa-GFP, a GFP-encoding sequence was integrated into the MFA locus. Opaque cells of SZ306a P_MFa-GFP were initially grown in liquid Lee's glucose medium (pH 6.8) to stationary phase. Cells were then reinoculated into fresh Lee's glucose medium (pH 5.0 and 7.0) and treated with α-pheromone at 25°C for 24 h.